EUROPEAN ARACHNOLOGY 2003 (LOGUNOV D.V. & PENNEY D. eds.), pp. 367374. © ARTHROPODA SELECTA (Special Issue No.1, 2004). ISSN 0136-006X (Proceedings of the 21st European Colloquium of Arachnology, St.-Petersburg, 49 August 2003) Glue droplets in fossil spider webs Ëèïêèå êàïëè íà èñêîïàåìûõ ïàóòèíàõ ïàóêîâ S. ZSCHOKKE Department of Integrative Biology, Section of Conservation Biology (NLU), University of Basel, St. Johanns- Vorstadt 10, CH-4056 Basel, Switzerland. email: [email protected] ABSTRACT. Amber is well known to conserve small insects and spiders, preserving the finest details of their morphology. However, spider webs in amber have so far largely been ignored, even though some webs in amber are superbly preserved; with even the smallest details being visible, like the glue droplets, which retain the prey in orb-webs and other araneoid webs. Here I present a brief introduction to the occurrence and function of glue droplets in Recent spider webs, followed by evidence of fossil glue droplets in Lebanese, Burmese, Baltic and Dominican ambers. A comparison of their states of preservation suggests that glue droplets in Lebanese amber have swollen up more than those in the other ambers. Finally, I discuss the evidence that these droplets are indeed fossilized glue droplets. ÐÅÇÞÌÅ. Õîðîøî èçâåñòíî, ÷òî ÿíòàðü ñîäåðæèò â ñåáå ìåëêèõ íàñåêîìûõ è ïàóêîâ, ñîõðàíÿÿ äàæå ñàìûå ìåëêèå äåòàëè èõ ìîðôîëîãèè. Îäíàêî, ïàóòèíà ïàóêîâ â ÿíòàðå äî ñèõ ïîð èãíîðèðîâàëàñü, íåñìîòðÿ íà òî, ÷òî íåêîòîðûå îáðàçöû ïàóòèíû âåëèêîëåïíî ñîõðàíè- ëèñü, ñ âèäèìûìè ìåëü÷àéøèìè äåòàëÿìè, òàêèìè êàê ëèïêèå êàïëè, êîòîðûå ñëóæàò äëÿ óäåðæàíèÿ äîáû÷è â ïàóòèíàõ àðàíåîèäíûõ ïàóêîâ. Çäåñü ÿ äàþ êðàòêîå ââåäåíèå â íàëè÷èå è ôóíêöèþ ëèïêèõ êàïåëü â ïàóòèíàõ ñîâðåìåííûõ ñåòåé ïàóêîâ, çàòåì ïðèâîæó äîêàçàòåëü- ñòâà ñóùåñòâîâàíèÿ ëèïêèõ êàïåëü â ëèâàíñêîì, áèðìàíñêîì, áàëòèéñêîì è äîìèíèêàíñêîì ÿíòàðÿõ. Ñðàâíåíèå êà÷åñòâà èõ ñîõðàííîñòè ïîçâîëÿåò çàêëþ÷èòü, ÷òî êëåéêèå êàïëè â ëèâàíñêîì ÿíòàðå áîëåå ðàçáóõøèå, ÷åì êàïëè â äðóãèõ ÿíòàðÿõ.  çàêëþ÷åíèå, ÿ îáñóæäàþ äîêàçàòåëüñòâà òîãî, ÷òî ýòè êàïëè äåéñòâèòåëüíî ôîññèëèçèðîâàííûå ëèïêèå êàïëè. KEY WORDS: Araneoidea, amber, glue droplet, spider silk, taphonomy, trace fossil, viscid silk, palaeontology. ÊËÞ×ÅÂÛÅ ÑËÎÂÀ: Araneoidea, ÿíòàðü, ëèïêèå êàïëè, øåëê ïàóêîâ, òàôîíîìèÿ, ñëåäîâîå èñêîïàåìîå, ëèïêèé øåëê, ïàëåîíòîëîãèÿ. Introduction the most interesting trace fossils are spider webs, parts of which are occasionally superbly pre- Amber (polymerized fossil tree resin) has served [Poinar & Poinar, 1999], sometimes down been known for a long time as a rich source of to what appear to be glue droplets of the viscid superbly preserved insects and other small or- silk [Bachofen-Echt, 1934; Zschokke, 2003]. ganisms [e.g., Poinar & Poinar, 1999; Weitschat Such droplets in fossil spider webs have & Wichard, 2002]. Until recently, however, been described several times. However, the trace fossils in amber, i.e., fossils not of the origin of these droplets has been interpreted in organism itself, but of something the organism different ways. The first description of spider left behind, have been largely ignored. Among webs in amber dates back to Menge [1856]. He 368 EUROPEAN ARACHNOLOGY 2003 Figs 13. Glue droplets in Recent webs: 1 Araneus marmoreus (Clerck, 1757); 2 Nephila sp.; 3 Cyclosa walckenaeri (O. Pickard-Cambridge, 1889). Scale: 500 µm. Ðèñ. 13. Ëèïêèå êàïëè â ñîâðåìåííûõ ïàóòèíàõ: 1 Araneus marmoreus (Clerck, 1757); 2 Nephila sp.; 3 Cyclosa walckenaeri (O. Pickard-Cambridge, 1889). Ìàñøòàá: 500 µm. interpreted these droplets as resin droplets that Recent glue droplets had flowed down along the silk thread. Almost a century later, Bachofen-Echt [1934] described Glue droplets are typically found in webs some spider webs with what he called Klümp- built by araneoid spiders. Indeed, the aggregate chen von Klebestoff (small clumps of adhe- silk glands, which produce the material for the sive, i.e., glue droplets). Wunderlich [1986: 10, gluey coating of sticky threads, have been sug- 227] figured two examples of spider silk with gested as the best single character defining the what he also considered to be glue droplets. superfamily Araneoidea [Coddington, 1986]. Schlee [1990] on the other hand discussed the The superfamily Araneoidea includes, among presence of small droplets along strands of others, all ecribellate orb-weavers and the fam- spider silk, considering them to be resin drop- ilies Linyphiidae and Theridiidae. In ecribel- lets that had flowed along the silk and which late orb-webs, glue droplets are found exclu- had dried to a certain extent before they were sively on the sticky spiral [e.g., Zschokke, 2002], embedded by a subsequent resin flow. Recent- where they are used to capture and retain prey. ly, Zschokke [2003] described a single thread In theridiid webs, glue droplets can be found with droplets, which he interpreted as glue drop- either along the gumfooted lines or within the lets. sheet, depending on the type of web [Benjamin No previous study has systematically docu- & Zschokke, 2003]. In these webs, glue drop- mented fossil droplets along strands of spider lets are also used to capture and retain prey. In silk embedded in amber. The aim of this paper contrast, in linyphiid webs, where glue droplets is to document droplets on spider silk in Leba- are found in the sheet, glue droplets are used to nese, Burmese, Baltic and Dominican ambers, cement the different layers of the sheet together and to examine the evidence that suggests that [Benjamin et al., 2002]. these droplets are indeed fossilized glue drop- Glue droplets in orb-webs consist of a com- lets. In the first section, I present some informa- plex mixture of glycoproteins, covered by an tion on Recent glue droplets. The second sec- aqueous solution of organic substances (mostly tion of the paper is devoted to the description of amino acids) and inorganic salts (KNO , 3 fossil glue droplets from the various ambers KH PO ) [for details see Vollrath et al., 1990; 2 4 and to the comparison of their state of preserva- Vollrath & Tillinghast, 1991], which makes tion. In the final section, I present evidence them highly hygroscopic [Townley et al., 1991]. supporting the hypothesis that these droplets In Recent orb-webs, the size of the glue droplets are fossilized glue droplets. A review of fossil varies considerably between species, with di- spider webs will be presented elsewhere. ameters ranging from less than 10 µm to 200 S. Zschokke. Glue droplets in fossil spider webs 369 Figs 46. Merging glue droplets of Nephila sp.: 4 after 0 seconds; 5 after 18 seconds; 6 after 40 seconds. Scale: 500 µm. Ðèñ. 46. Ñëèÿíèå ëèïêèõ êàïåëü ó Nephila sp.: 4 ÷åðåç 0 ñåêóíä; 5 ÷åðåç 18 ñåêóíä; 6 ÷åðåç 40 ñåêóíä. Ìàñøòàá: 500 µm. µm in undisturbed webs [Figs 13; Peters, 1987; Fossil glue droplets Opell, 2002]. The largest glue droplets are prob- ably found in the webs of some Nephila spp. Here, I present data of fossil spider silk with (Tetragnathidae) [pers. obs.]. In linyphiid webs, what I consider to be fossil glue droplets (see below) from four different types of amber, ordered by age. droplet size also varies greatly, however, these The uniformity of the size of the glue droplets within droplets seem to be smaller than those in orb- each specimen was assessed by calculating their webs [210 µm diameter; Peters & Kovoor, coefficient of variation (C.V.). The following abbre- 1991; Benjamin et al., 2002]. For some species, viations for collections are used in the text: JW = it is characteristic for the glue droplets along a Jörg Wunderlichs collection (Hirschberg-Leuters- thread to alternate in size between large and hausen, Germany); MEW = Museum of the Earth, small ones [Fig. 1; Vollrath, 1992]. When an Warsaw, Poland (Dr. J. Kupryjanowicz); SMNS = Staatliches Museum für Naturkunde, Stuttgart, Ger- orb-web is damaged and threads with glue drop- many (Dr. G. Bechly); SZ = authors personal col- lets have come into contact with each other, lection. Absolute ages are based on the IUGS [2000]. these threads will tend to slowly merge into one thread, which then bears droplets of increased Lebanese amber size and a less regular arrangement (Figs 46). Lebanese amber collected near Jezzine is This merging is most likely caused by the sur- considered to be the oldest amber with animal face tension of the droplets. inclusions and is dated to the late Valanginian to 370 EUROPEAN ARACHNOLOGY 2003 Table. List of amber specimens containing spider silk with glue droplets. Median droplet size and range (in parentheses) is given, as well as the Coefficient of Variation (C.V.) and the number of droplets. Òàáëèöà. Ñïèñîê îáðàçöîâ ÿíòàðÿ, ñîäåðæàùèõ ïàó÷üþ ïàóòèíó ñ ëèïêèìè êàïëÿìè. Äàíû ñðåäíèé ðàçìåð êàïëè è âàðèàöèè (â ñêîáêàõ), à òàêæå Êîýôôèöèåíò Âàðèàöèè (C.V.) è êîëè÷åñòâî êàïåëü. Amber type Age Botanical Specimen Droplet size C.V. # droplets (Mya) source (µm) Lebanese 130135 Agathis levantensis SMNS: 19/2 12 (785) 1.02 38 (Coniferales: Araucariaceae) Burmese 95105 Metasequoia sp. SZ: M 5 40 (13275) 0.84 65 (Coniferales: Cupressaceae) SZ: M 15 41 (17116) 0.61 22 Baltic 3550 Pinites succinifer or SZ: B 14 119 (42385) 0.60 31 Pseudolarix sp. SZ: B 18 19 (855) 0.57 42 (Coniferales: Pinaceae) MEW: 20493TG 19 (9113) 0.77 109 Dominican 1520 Hymenaea protera SZ: D 38 25 (8456) 1.26 102 (Fabales: Fabaceae) SZ: D 40 129 (291250) 1.16 78 SZ: D 41 76 (25778) 1.15 43 SZ: D 46 51 (17574) 1.30 45 SZ: D 47 30 (12812) 1.62 28 Hauterivian stage of the Lower Cretaceous (c. only 22 droplets are visible, with diameters 130135 Mya = Million years ago) [Schlee & ranging from 17 to 116 µm. Dietrich, 1970]. The botanical origin of Leba- nese amber is thought to be the Kauri pine (Ag- Baltic amber athis levantensis, Coniferales: Araucariaceae) Baltic amber constitutes most of the amber [Lambert et al., 1996; Poinar & Milki, 2001].